I think I find the cause of the problem. The form tag uses in the reply/edit/new posting body use
enctype="multipart/form-data". And this results in the browser does not use the utf-8 encoding even accept-charset="utf-8" is used.
Google for the whole day did not find any solution. One work around, but with sacrifice, for this case is to use the default enctype, i.e. enctype="application/x-www-form-urlencoded" in form tag. The sacrifice is not able to use the file upload in the same form.